The present invention pertains to transformers, and, in particular, to an apparatus and method for cooling power transformers during use.
Power transformers are employed within power supply systems in order to transform, transmit and distribute electricity for end-user consumption. Transformers are designated by high and low side operating voltages, and sized as to capacity of the volts and amperes being carried. For example, large-size transformers are utilized as transmission transformers, which step up the voltage along the power supply chain, as well as distribution transformers, which step down the voltages for distribution.
One shortcoming of existing transformers is their susceptibility to operational problems associated with high temperatures of operation, both internal and external to the transformers. Typically, in order to maintain rated capability and preserve useful life of the transformer and all of its constituent parts, maximum temperature within the transformer should be maintained below the lesser of 95° C. (203° F.) and a temperature that is 65° C. above ambient temperature. The failure to keep the transformer temperature so regulated can result in failure of the transformer or perhaps the significant reduction of its useful life, each of which results in high cost to the industry due to the need to replace the destroyed transformer units.
Moreover, due to the directly proportional relationship between temperature and electrical resistance, when the temperature of the copper windings in the transformer core increases, the efficiency of the transformer decreases, thereby resulting in a loss of power output (watts) proportional to transformer core heating. Moreover, during use, the temperature inside the transformer tends to increase due to the electrical current flowing through the conducting windings and the micro-current flowing in the magnetic steel core.
Some prior attempts at controlling transformer temperature have been relatively crude. For instance, one common approach has been to simply drench the transformer with a water spray when ambient conditions suggest the risk of excessive transformer temperature, or when a high temperature condition is sensed.
In another approach, oil baths have been provided for the inner workings of the transformer. In different prior art applications, such oil baths were designed to operate on several levels. First, a “self cooled” level essentially relies on convention currents within the transformers insulating and cooling oil to draw heat away from the core. A second level uses a forced circulation of the insulating oil through heat exchangers/radiators integral with or separate from the transformer which utilize ambient air around the heat exchangers to absorb the heat energy of the cooling oil. A third level uses the forced oil circulation of the second level but adds electric fans, powered by energy supplied from the transformer itself or other sources of power in the substation, to force air circulation over the external radiators thus increasing heat removal from the oil and therefore the transformer windings, and thereby increasing transformer efficiency. These fans, which are selectively operated when transformer temperature rises are sufficiently large, are controlled by a controller connected to temperature sensors located in and on the transformers.
A prior art system that uses fans is diagrammatically shown in FIG. 1. The transformer, generally designated 10, is of conventional design and includes a casing or housing in which is disposed a soft iron core 12 with copper windings 14 there-around. The core and windings are immersed in a bath of cooling oil 15. A nitrogen gas blanket 16 at the top of the internal volume of the transformer housing maintains the quality of the oil within the housing.
Positioned proximate the top of the transformer housing is an outlet connected via a top isolating valve 18 to a conduit 20 that leads to a radiator or heat exchanger, generally designated 22. In this prior system, radiator 22 includes finned cooling tubes 24 through which the cooling oil is circulated. The tubes are oriented in a series of spaced apart rows and columns to allow the passage of ambient air there-around for cooling purposes. A plurality of motor-driven fans 26 are designed to draw air over and around the finned cooling tubes 24 in order to provide forced-air ambient cooling. The outlet of radiator 22 is plumbed to a sealed, motor-driven pump 28 that pumps the cooling oil through conduit 30, bottom isolating valve 32, and back into the internal volume of the transformer housing.
During operation, pump 28 forces cooling oil into the base of the transformer as indicated by arrow 33. As the oil travels upward, as indicated at 35, over and through the various openings provided within the internal workings of the transformer (such as the core 12 and windings 14), the temperature of the cooling oil increases as it draws off heat, and thereby cools the transformer parts which have increased in temperature due to their operation. The now heated oil passes through the oil outlet at 37 into conduit 20 and is routed through radiator 22. Ambient air, which is ducked into the area in which the radiator is installed, is drawn over the cooling tubes 24 by fans 26 to cool the oil passing through tubes 24. The ambient air which has been heated as the energy is drawn off the cooling oil is discharged to the atmosphere, and the cooled fluid is returned to pump 28 for recirculation through the transformer.
While the prior art cooling system shown in FIG. 1 does provide some benefit, its cooling limitations result in some transformers being operated in conditions which are undesirable. Specifically, limits imposed by ambient conditions, most specifically temperature humidity, can result in the cooling oil passing entirely through the heat exchanger without a sufficient removal of heat energy, such that over time the temperature of the cooling continues to build, and the cooling capabilities of such oil then declines. In the end, the cooling oil may become too hot to prevent the transformer from exceeding recommended temperatures.
Also, transformer utilization and consequently load current loading typically occurs during the highest ambient temperature conditions. For example, the temperature of the oil bath may be elevated on days when the ambient conditions are extremely warm and humid, and consequently the oil is not adequately cooled and the temperature continues to build in the transformer until damage can be done to the transformer workings.
Thus, it would be desirable to provide a cooling system for power transformers which overcomes these and other shortcomings of the prior art.